Full Minimal Coupling GW-Bethe-Salpeter-Equation Framework for Circular Dichroism in Solids: Applications to Chiral...

Circular dichroism (CD) and other chiroptical responses are key probes of both chirality and momentum-space geometry in solids, but first-principles calculations are still challenging in periodic systems with strong exciton effects. Here, we develop a gauge-invariant first-principles framework for CD including exciton effects based on full minimal coupling (FMC) within the GW plus Bethe-Salpeter equation (GW-BSE) formalism. In contrast to standard multipole expansion and sum-over-states (SOS) approaches, which require careful gauge fixing, converge slowly, and suffer from origin ambiguities, FMC evaluates optical matrix elements directly at finite photon wave vector, naturally including intraband and near-degenerate transitions while placing electric dipole (ED), magnetic dipole (MD), and electric quadrupole (EQ) contributions on equal footing. Applied to two prototypical two-dimensional chiral hybrid perovskites, (S-NEA)_{2}PbBr_{4} and (S-MBA)_{2}PbI_{4}, our calculations reveal that MD and EQ channels both contribute to the CD signal. Crucially, intraband and quasidegenerate transitions captured only within FMC can significantly modify CD spectra, especially in systems with dense band degeneracies. The FMC framework, therefore, offers a computationally efficient and numerically robust way for predicting chiral optoelectronic phenomena in complex solids.